1. Field
This disclosure relates to a method and apparatus for the excitation and transmission of highly nonlinear pulses with selectable pulse properties into a structure or material and the detection of such pulses from the structure or material. More particularly, the present disclosure describes a method and apparatus for exciting a selectable number of controllable highly nonlinear pulses with desired shapes, amplitudes, frequencies and/or durations, which may then be used for nondestructive evaluations and/or structural health monitoring.
2. Description of Related Art
Non-destructive evaluation of a material or structure may be accomplished through the use of impact testing. In impact testing, the material or structure is typically struck with an impact device and sound waves propagating through the material or structure are then measured to provide some indication of defects within the material or structure. See, for example, U.S. Pat. No. 5,165,270 to Sansalone, et al., dated Nov. 24, 1992. In U.S. Pat. No. 5,165,270, the impact device is a number of differently weighted spheres that are each designed to produce a different duration of impact, thereby imparting different stress waves into the structure to be tested. The different stress waves have different frequency values depending on the impact duration. Each sphere is disposed on one end of a spring-steel rod. At the start of the test, a selected sphere is in a resting position. The sphere is withdrawn from the rest position by a pair of jaws to a given height above the structure. This action deflects the spring-steel rod, thus increasing the potential energy of the impact sphere. At a predetermined release point, the sphere is released causing it to impact the structure and impart a given energy to the structure. The impact produces stress (sound) waves that are reflected from the external surfaces and/or internal defects of the structure. The reflected waves are detected by a transducer that converts the normal surface displacements caused by the waves into an electrical signal. The electrical signal is then processed to provide an amplitude/frequency spectrum indicative of either the thickness of the structure or the defects disposed therein.
Other impact testing apparatus and techniques are known in the art, but generally use approaches similar to that described above, i.e., strike the material to be tested and measure the stress wave propagation. The impact devices (i.e., strikers) used in impact-testing technology typically cost several hundreds of dollars or more and need coupling to a signal conditioner. Line-powered signal conditioners are used to power sensors and condition their output signals for transmittal to readout and recording instruments. Impact hammers are used for delivering impulse forces into test specimens and the signal conditioner is used to provide electrical measurement signals of the amplitude and frequency content of the applied force. Hammers and conditioners used for non-destructive evaluation may be very expensive. Embodiments of the present invention as described below may provide for less costly apparatus for nondestructive evaluation of materials and structures.